Many process parameters in the manufacture of an IGFET can result in variations in the threshold voltage, V.sub.th. The threshold voltage, V.sub.th, characterizes the voltage at which conduction between the source and the drain (inversion) is abruptly enhanced. In effect, this is the gate voltage required to neutralize the immobile charge above and below the channel region so that any further increase in gate voltage mobilizes minority carriers in the channel region. A more thorough analysis of V.sub.th is given in MOSFET IN CIRCUIT DESIGN by Robert H. Crawford, McGraw-Hill Book Company, 1967, pages 21-26 and pages 37-40.
In our experience, variations in V.sub.th are typically less than 1.5 volts between wafers. This variation has undesirable effects upon the design of IGFET integrated circuits since the circuits must be designed to operate for the entire range of V.sub.th. Circuits designed to accommodate a large variation in V.sub.th are characterized by high power consumption, wasted area and high current densities.
Accordingly, the principal object of the present invention is to bias the substrate voltage, V.sub.BG in response to a variation in V.sub.th such that V.sub.BG becomes more negative as the intrinsic (V.sub.BG =0) V.sub.th decreases. A further object of the present invention is to reduce the variation in V.sub.th due to process parameters in the manufacture of IGFETs and thereby permit the design of more optimized IGFET integrated circuits. Another object of the present invention is to improve the long term stability of IGFET integrated circuits by reducing threshold voltage shift due to aging. Other and incidental objects of the present invention will become apparent from a study of the following detailed description.
These objects are accomplished in accordance with the preferred embodiment of the invention by sensing the threshold voltage of an IGFET and regulating a voltage divider circuit to adjust the voltage of the substrate relative to ground. Varying the substrate voltage, also known as a backgate voltage, V.sub.BG, varies the threshold voltage as described by Crawford, supra, pages 40-45. By appropriate circuit design, a large range of threshold voltages can be compensated to a near nominal value.